As 'Big Bertha' gets ready to dig, plenty of questions remain

SEATTLE -- Big Bertha the boring machine recently arrived to start digging the Alaskan Way Viaduct replacement tunnel, and many questions remain about how the massive project will unfold.

Tom Soran knows a thing or two about digging tunnels. Some 20 years ago he helped design and engineer the boring machines used to dig the Chunnel between England and France.

That project is considered a huge success now, but at the time it hit snag after snag and eventually ran 80 percent over budget.

Soran said he sees two potential pitfalls Big Bertha could face while digging the viaduct replacement tunnel. The first is that Bertha will be cutting through sand, dirt and rock.

"That's your main problem you have to address, and the second problem is how to get rid of that," he said.

Soran said Bertha's cutter at the front and conveyor belt designed to carry dirt out the back are key pieces that make or break any tunnel project.

"We've got really poor soils up and down Alaskan Way," said Linea Laird with the Washington Department of Transportation.

WSDOT officials say they've taken soil samples and expect Bertha will hit a lot of thick clay during her 2-mile, year-and-a-half journey underneath Seattle.

By looking at the size of the holes in the cutter, Soran said he thinks WSDOT is expecting a lot of different types of dirt along the way. He hopes it's not as bad as he experienced during the Chunnel dig.

"Seawater was coming in there and water would run around the tunnel boring machine," he said.

Soran believes if Bertha takes it slow and her crew adjusts to problems quickly, she'll do just fine.

"At the end of the day you look back and see what you did and I was very satisfied with what was accomplished," he said.